1. Field of the Invention
This invention relates to radar test devices and, more particularly, to radar test sets which are generally co-located with the radar and produce a substantially delayed replica of the transmitted radar signal to simulate reflection of the radar signal off of a distance target.
2. Description of Related Art
Overall (end-to-end) radar system performance can be evaluated by reflecting the radar signal off of a target located a great distance from the radar. This great distance, unfortunately, creates several problems.
One problem is that it is often difficult to accurately point the radar at the test target. Another problem is that the radar signal often is reflected off of other objects, in addition to the test target, creating undesirable "clutter".
A still further problem is that such tests often need to be performed with signals having relatively high power in order to obtain a return signal of adequate strength. And a still further problem is that it is often difficult to coordinate efforts when part of the test system is located a great distance from the other part.
In order to avoid these problem, numerous efforts have been made to design radar test targets which, although co-located with the radar, contain a device which will nevertheless produce a delayed replica of the radar signal.
One such effort uses a co-located wave guide. Unfortunately, the only practical use for such a device is to create short delays. A long delay would require a long waveguide which would be bulky, inflexible and would generate large losses.
Another effort has been to use long coaxial cables to create the delay. Unfortunately, this technique causes very high and undesirable losses when a long delay is produced. The coaxial cable is also very bulky, making it difficult to easily switch in a cable having a different length when a different delay is desired.
A still further technique is to mix the radar signal with a much lower frequency carrier, transmit that mixed carrier through a digital, quartz, or surface acoustic wave (SAW) delay line, and to then re-mix the delayed signal back to radar frequency.
Such mixing efforts, however, have been far from ideal. The mixing is usually inefficient, adding noise and other types of errors. Further, such techniques generally result in high losses. They also usually require reference and timing signals, making the test target a fairly complex, cumbersome, and expensive system.
In short, there has been a continuing need for a radar test target which can be co-located with the radar and which will produce a faithful replica of the radar signal, substantially delayed in time.